1. Preheat oven to 350°F. Line baking sheets with parchment paper or leave ungreased.2. Combine flour and salt in a bowl; set aside. 3. Combine almonds and 1/3 c. of sugar in a food processor fitted with a metal blade. Pulse until finely ground. (Be careful not to grind to a paste).4. Combine butter and remaining sugar in a medium bowl. Cream together until light and fluffy. Add ground almond and sugar mixture. Mix to combine. Add vanilla and egg yolks. Mix to combine. Mix in flour.5. Roll dough into walnut-size balls and place 2 in. apart on baking sheets. Dip your thumbs in flour and use to make a deep indentation in center of each cookie. 6. Whisk egg whites to loosen them and then brush outside of each cookie with egg white. Scatter a few chopped almonds on outer edge of each cookie if desired. Fill each center with 1/2 tsp. of jelly. 7. Bake until edges are golden brown, 15 to 18 min. Remove from oven. If needed, immediately add another 1/4 tsp. of jelly to each cookie. Let stand for 3 to 4 min. Transfer to cooling racks with a thin metal spatula. Dust with confectioners' sugar if desired.